Firefox 23 release notes for developers

Changes for web developers

Developer Tools

A Network Panel has been added to the developer tools. This is more detailed view than the "Net" view option in the Web Console.
Web Console has been renamed "Console", and includes the option to filter security errors/warnings.
The new Toolbox Options lets you disable features, change light/dark theme, or enable Chrome/Remote debugging.

HTML

The <blink> element support is now completely dropped. The <blink> tag now implements the {{domxref("HTMLUnknownElement")}} interface (Firefox bug 857820).
The range type of the {{HTMLElement("input")}} element (<input type="range">) has been switched on by default (Firefox bug 841950).

JavaScript

The Object.defineProperty method can now be used to redefine the length property of an Array object.
The option to disable JavaScript, including the options to allow moving windows/replace context menu, have been removed. You may still disable JavaScript by double clicking the "javascript.enabled" option in about:config.

DOM

D3E KeyboardEvent.key is now supported, but only for non-printable keys (Firefox bug 842927).
The title attribute of {{domxref("DOMImplementation.createHTMLDocument")}} is now optional as per updated DOM specification.
The ability to add a sidebar panel (window.sidebar.addPanel) has been dropped (Firefox bug 691647).
The unprefixed {{domxref("Window.requestAnimationFrame")}} and {{domxref("Window.cancelAnimationFrame")}} methods has been added (Firefox bug 704063). The unprefixed {{domxref("Window.requestAnimationFrame")}} receives a {{domxref("DOMHighResTimeStamp")}} as argument; the prefixed version receives a timestamp in milliseconds (Firefox bug 753453).
The text argument for {{domxref("window.alert")}} and {{domxref("window.confirm")}} is now optional (Firefox bug 861605).
The HTMLMediaElement.initialTime property, removed from the spec, is no longer supported (Firefox bug 742537).
The {{domxref("AnimationEvent.AnimationEvent", "AnimationEvent()")}} constructor has been added (Firefox bug 848293).
The {{domxref("AnimationEvent.pseudoElement")}} property has been implemented (Firefox bug 848293).
The {{domxref("TransitionEvent.TransitionEvent", "TransitionEvent()")}} constructor has been added (Firefox bug 848291).
The {{domxref("TransitionEvent.pseudoElement")}} property has been implemented (Firefox bug 848291).
The non-standard TransitionEvent.initTransitionEvent() and AnimationEvent.initAnimationEvent() have been removed (Firefox bug 868751).

WebRTC

Instead of including usernames in the RTCIceServer.url property (such as stun:[email protected]), you now need to use the new RTCIceServer.username property.

CSS

The blink effect for text-decoration: blink; has no more effect, but is still a valid value (Firefox bug 857820).
In-flow {{cssxref("::after")}} and {{cssxref("::before")}} pseudo-elements are now flex items (Firefox bug 867454).
The way to compute viewport units has been changed. In conjunction with overflow:auto, space taken by eventual scrollbars is not subtracted from the viewport, whereas in the case of overflow:scroll, it is. (Firefox bug 811403)

MathML

Negative widths for the {{MathMLElement("mspace")}} element has been implemented (Firefox bug 717546).
The {{MathMLElement("semantics")}} element now determines the visible child as described in the MathML3 specification.

Security

Mixed content blocking. Firefox will no longer load non-secure (http) resources on secure (https) pages (Firefox bug 834836).
The standard syntax of CSP 1.0 policies are now implemented and enforced by default.

Changes for addon and Mozilla developers

Firefox developer tools

Addons that overlay chrome://browser/content/debugger.xul must now overlay chrome://browser/content/devtools/debugger.xul. You may add references to both these files in chrome.manifest for compatibility.